Size: The size of the crystal can greatly determine the purity of the crystal. This is due to the complete order of the lattice structure. One can see the inside points of the lattice structure are very precise dimensions and can only have the correct atoms at those points. You may also see that the edge points have usually only 3 to 5 bonds (Corners, Edges, and Sides). Because of this, there is a greater chance of an impurity being able to fit in one of these spaces. Therefore the purity a function of the internal volume (all of which is very pure) and the surface area (Corners, Edges, and Sides) which have lower purity. To maximize purity, one would therefor want the greatest internal volume with the least amount of surface area.

Process by which crystals are formed: The rate at which the crystals grow determines whether or not the crystals will have a large or smaller number of internal impurities. This is better explained here.

Wash method or recrystallization: Typically a crstyal is washed after it is formed to remove many of the surface impurities. When selecting a solvent to use for the washing step in you crystallization process, one must consider solubility of the crystal as well as solubility of the impurities. A good choice of solvents is ice water. This is due to the temperature dependance of solubility for water. See this example for temperature effects. So crystals, such as silicon are melted and recrystallized slowly to increase the selectivity of the surface atoms towards the desired products.